Four days after Hurricane Sandy slammed New York, a huge chunk of New York’s subway remains closed. That carries a big cost -- commutes up to four hours each way, lost business. Experts say the cost to the economy could run to the hundreds of millions.

And they say the threat of rising sea levels coupled with big storms like Sandy’s to the city’s subway system -- and its economy -- was both predictable and predictable.

On Monday night, as Sandy’s storm surge hit, saltwater rushed into all five subway tunnels linking Manhattan with Brooklyn.

More people ride through those tunnels then ride through every other transit system in America. MTA Chief Joe Lhota described the scene the next day. “The MTA faced a disaster as devastating as it has ever faced in its history," he said.

The MTA had been aware of the danger.

About a year ago I spoke to Colombia University’s Klaus Jacobs. He modeled a storm like Sandy and brought his findings to the MTA. “And there was a big silence in the room,” Jacob said. “Because the system is so old. Many of the items that would be damaged by the intrusion of the saltwater into the system could not recover quickly.”

That’s a prediction that came eerily close to reality. Without power, pumping out the tunnels is slow. MTA officials need to dry out the parts, and then check each of them before fully opening the subway.

More than a year ago, Jacobs produced a subway with the flooded lines colored in deep blue, looking like skeletal fingers under the East River. Last night, the MTA released its own map of the new subway. The closed lines almost exactly reflect Jacob’s model.

By Thursday, none of the five flooded tunnels under the East River had reopened. In the subway, announcers intoned: “Ladies and Gentlemen, there is no train service between Brooklyn and Manhattan.”

Officials set up shuttle buses to replace the subways, but the transfers were choked. A line at Brooklyn’s brand new basketball arena, BarclaysCenter, stretched fully around the arena -- the same day the Arena was to host the Nets v. the Knicks season opener.

New York’s governor, Andrew Cuomo, says New York has to rebuild its subway system to make it less vulnerable to storms like Irene and Sandy, which hit New York just 14 months apart. “Part of learning from this is the recognition that climate change is a reality, extreme weather is a reality, it is a reality that we are vulnerable,” Cuomo said.